as the title reads, there is a rare find in besaid from the woman at the place where you can buy items. What is it and what does it do, I haven't been able to purchase it. So what is it and is it worth it?

It's nothing really, it's a key and you can get it when you rescue all the people in the Sunken Cave mission, it lets you open a chest in the Besaid temple with a serarch sphere wich you use to locate spheres on besaid that gve you a new code to put on the cave

Yeah, don't worry about buying, as was said, you get it as a freebie later. Of course, in New Game Plus, you could possibly have enough money to buy it, but there's still no point.

yea thats just the key, you well find it later. I wasted the money for the key early on in the game :P

I've never boyght it so what do you get from the man that you rescue in the sunken cave?? nothing??? well...

You mean what do you get from the dude in the cave if you buy it instead? Yeah, nothing, I'd imagine. After all, not everybody you rescue from that cave gives you something.